The present invention relates to surgical apparatus and methods for the treatment of tissue by application of light energy, and particularly to the area of laser surgery wherein tissue is welded or bonded together under controlled temperature conditions.
According to well known surgical techniques, tissue is connected together with suturing materials such as silk, synthetic thread, or metal staples, and then allowed to permanently bind together by the normal healing process. One of the drawbacks, however, of joining tissue with suturing materials is the introduction of foreign material into the tissue. The foreign material may cause intimation, infection and scarring. Additionally, bonding tissue together with suturing material does not create a fluid tight seal. Laser welding tissue together, in comparison, creates a fluid tight seal and does not introduce foreign material into the tissue.
A variety of laser systems have been used for welding tissue, including, Argon, milliwatt CO.sub.2, and neodymium doped yttrium aluminum garnet (Nd:YAG) lasers. Each of these lasers is generally understood to cause tissue heating that produces structural changes in tissue, thereby causing cross-linking of proteins and binding of the tissue. As currently practiced, the laser is directed onto the tissue for a time period and with a specified power until subjective tissue changes occur, such as blanching, browning, or shrinking.
Relying on subjective factors to determine the appropriate exposure time and power setting requires considerable experience and instinct. Furthermore, once these changes in the tissue become apparent (i.e. blanching, browning, or shrinking), some damage to the tissue has already occurred. An advantageous system would weld the tissue together without causing the tissue temperature to rise to a burning point.
Another drawback to laser welding as currently practiced, is the failure to match laser wavelengths to respective tissues in order to achieve full absorption of the laser energy with the resulting increased weld strength. For example, Rink et al., U.S. Pat. No. 5,269,778, discloses a variable pulse width laser for ablating the surface of the target tissue without affecting the underlying tissue. As a result, when the laser penetration depth is less than the depth of the selected tissue the laser energy is deposited superficially on the selected tissue, causing excessive heating and possibly charring at the outer tissue surface, and incomplete fusion at further depths. Alternatively, when the laser penetration depth, or optical absorption depth, exceeds the depth of the selected tissue only a small fraction of the energy from the laser is absorbed by the tissue. This creates a need for higher laser intensities and longer exposure times to attain a suitable fusion temperature, in addition, any energy not deposited at the weld site may result in uncontrolled damage to surrounding tissue.
There are further drawbacks to current laser welding systems. For example, the wavelength of the milliwatt CO.sub.2 laser is too long to allow the laser energy to be transmitted via silica fiber optics. Instead, a series of mirrors in an articulating arm is required which is cumbersome and difficult to use. Other systems utilize Argon lasers, but these lasers generate high temperatures in the surrounding tissues that require continuous saline irrigation to maintain a narrow welding temperature range and prevent tissue desiccation. As a result, both the CO.sub.2 and Argon laser welding systems require complicated and expensive machinery and a great deal of training to implement.